Evaporation installations are frequently employed for the purification of liquids, particularly radioactive waste water. The distillate resulting from such installations are ordinarily conducted from the condensing stage thereof to a final degassing stage to remove the remaining volatile impurities therefrom.
In order to obtain an effective degassing effect, it is necessary that the temperature of the distillate to be conducted to the degassing stage be at or slightly below its boiling point. Since the temperature at the outlet point of the condensing stage is, for economic reasons, generally substantially below its boiling point, preheating of the distillate prior to the degassing is required.
Such preheating can be obtained in various known ways, all of which require substantial additional amounts of heat as well as a relatively large and expensive technical plant. It is known to preheat common water to be degassed using a heat exchanger heated by steam. The temperature of such water at the entrance of the heat exchanger is relatively constant. Therefor it is not difficult to control such a preheater. However, because of the great difference between the heat of condensation of water and the specific heat thereof, any slight unavoidable change in the amount of the condensing vapor leads to a strong change in the temperature of the distillate. Such strong temperature oscillations in the distillate cannot be totally removed and can only be diminished through highly-sensitive and correspondingly expensive and disturbance-susceptible control arrangements. So it is much more difficult to control preheating of distillate than of common water. Moreover, such arrangement must obviously be laid out for the lowest possible temperature of the distillate, which creates additional expense.